Brake rotor thickness requirements vary by vehicle make, model and year, but the minimum allowable thickness is always cast on an unmachined surface, or alternate part, of the rotor. A technician must remove the wheel and use a micrometer on the rotor to measure its thickness accurately.
The technician must remove any foreign material buildup from the rotor before taking a measurement. The micrometer needs to be 0.4 inch inside of the outer rotor circumference to obtain the correct measurement. The technician takes subsequent readings every 45 degrees, or one-eighth turn, of rotation around the entire rotor to allow for inconsistent wear. When refinishing a rotor, a correct rotor runout reading is also necessary to calculate its minimum necessary thickness.